A primary immunodeficiency is a disorder in which part of the immune system is missing or does not function normally. To be classified as primary, the cause of the immunodeficiency must not be another disease, drug therapy, or exposure to toxins. Many primary immunodeficiencies are genetic disorders, and many people are born without some of the immune system's defenses, making them susceptible to germs that can cause infections. Immunodeficiencies weaken the immune system, which makes infections and other problems more likely to occur. Some types of immunodeficiency are so mild that they can go undetected for years. Some types are so severe that they are detected even in newborn babies